1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the same.
2. Description of the Related Art
In accordance with miniaturization of the silicon semiconductor integrated circuit, the size of an MIS (Metal Insulator Semiconductor) transistor is rendered smaller and smaller. According to ITRS (International Technology Road map for Semiconductors), 2000 edition, the technology nodes of 60 nm require EOT (Equivalent Oxide Thickness), which is a thickness of the gate insulator converted into the thickness of a silicon oxide film based on the dielectric constant, falling within a range of between 0.8 nm and 1.2 nm. However, if EOT is set to fall within the range noted above and a silicon oxide film or a silicon oxynitride film is used as a gate insulator, it is impossible to suppress sufficiently the leak current. Therefore, it is necessary to use an insulating film with a high dielectric constant, i.e., a high-k film containing metal, as the gate insulator.
In recent years, vigorous research has been conducted on, for example, Ta2O5, TiO2, Al2O3, ZrO2, HfO2, Zr silicate (ZrSiOx) and Hf silicate (HfSiOx) as a material of the next generation gate insulator with a high dielectric constant. Particularly, ZrO2, HfO2 and silicates thereof are high in the thermodynamic stability on an Si substrate, have a high dielectric constant and a large band gap and, thus, are considered to be particularly hopeful as a material of the gate insulator of the sub-1 nm generation.
However, the following problems are pointed out in respect of the thermal stability in the interface between the ternary insulator such as M—Si—O (M═Zr, Hf) and the Si substrate.
The first problem is derived from the situation that oxidizing species such as O2 and H2O have a relatively high diffusion rate within the particular insulator. If the oxidizing species have a high diffusion rate within the insulating film, traces of the oxidizing species contained in the atmosphere are readily migrated through the insulating film during various heat treatment steps, with the result that a thick SiO2 film is formed at the interface between the insulator and the Si substrate. The formation of the SiO2 film lowers the dielectric constant of the gate insulator so as to increase EOT.
The second problem is brought about in the case where the partial pressure of the oxidizing species within the heat treating atmosphere is lowered in an attempt to prevent the SiO2 film from being formed. Specifically, if the structure of an insulating film/Si substrate is subjected to a heat treatment at a temperature not lower than 900° C. under UHV (Ultra High Vacuum) in which the partial pressure of the oxidizing species is lowered, it has been confirmed that a metal silicide (MSix) is produced at the interface between the high-k film and the Si substrate, which brings about degradation of the morphology. Incidentally, the particular reaction takes place not only at the interface between the high-k film and the Si substrate but also at the interface between the high-k film and a polycrystalline silicon (poly-Si) gate electrode or a polycrystalline silicon germanium (poly-SiGe) gate electrode.
As described above, in order to suppress the formation of an SiO2 film at the interface between the high-k film and the Si substrate, it is necessary to suppress the partial pressure of the oxidizing species to a low level in the atmosphere. However, if the partial pressure of the oxidizing species is excessively lowered, a silicide is formed. Therefore, in the case where an Si substrate having a high-k film formed thereon is subjected to a heat treatment step, it is necessary to control the partial pressure of the oxidizing species in the atmosphere to fall within a prescribed range in order to suppress both formation of an SiO2 film and silicide.
However, the partial pressure range of the oxidizing species in which formation of an SiO2 film and a silicide can be suppressed is very narrow, which makes it very difficult to control the partial pressure of the oxidizing species to fall within the desired range. This raises a serious obstacle in applying a high-k film to the present semiconductor process, which includes many heat treatment steps at high temperatures such as an activation anneal.